The terms urate oxidase and uricase are used herein interchangeably. Urate oxidases (uricases; E.C. 1.7.3.3) are enzymes which catalyze the oxidation of uric acid to a more soluble product, allantoin, a purine metabolite that is more readily excreted. Humans do not produce enzymatically active uricase, as a result of several mutations in the gene for uricase acquired during the evolution of higher primates. Wu, X, et al., (1992) J Mol Evol 34:78-84, incorporated herein by reference in its entirety. As a consequence, in susceptible individuals, excessive concentrations of uric acid in the blood (hyperuricemia) can lead to painful arthritis (gout), disfiguring urate deposits (tophi) and renal failure. In some affected individuals, available drugs such as allopurinol (an inhibitor of uric acid synthesis) produce treatrnent-limiting adverse effects or do not relieve these conditions adequately. Hande, K R, et al., (1984) Am J Med 76:47-56; Fam, A G, (1990) Bailliere's Clin Rheumatol 4:177-192, each incorporated herein by reference in its entirety. Injections of uricase can decrease hyperuricemia and hyperuricosuria, at least transiently. Since uricase is a foreign protein in humans, even the first injection of the unmodified protein from Aspergillus flavus has induced anaphylactic reactions in several percent of treated patients (Pui, C-H, et al., (1997) Leukemia 11:1813-1816, incorporated herein by reference in its entirety), and immunologic responses limit its utility for chronic or intermittent treatment. Donadio, D, et al., (1981) Nouv Presse Med 10:711-712; Leaustic, M, et al., (1983) Rev Rhum Mal Osteoartic 50:553-554, each incorporated herein by reference in its entirety.
The sub-optimal performance of available treatments for hyperuricemia has been recognized for several decades. Kissel, P, et al., (1968) Nature 217:72-74, incorporated herein by reference in its entirety. Similarly, the possibility that certain groups of patients with severe gout might benefit from a safe and effective form of injectable uricase has been recognized for many years. Davis, F F, et al., (1978) in G B Broun, et al., (Eds.) Enzyme Engineering, Vol. 4 (pp. 169-173) New York, Plenum Press; Nishimura, H, et al., (1979) Enzyme 24:261-264; Nishimura, H, et al., (1981) Enzyme 26:49-53; Davis, S, et al., (1981) Lancet 2(8241):281-283; Abuchowski, A, et al., (1981) J Pharmacol Exp Ther 219:352-354; Chen, R H-L, et al., (1981) Biochim Biophys Acta 660:293-298; Chua, C C, et al., (1988) Ann Int Med 109:114-117; Greenberg, M L, et al., (1989) Anal Biochem 176:290-293, each incorporated herein by reference in its entirety. Uricases derived from animal organs are nearly insoluble in solvents that are compatible with safe administration by injection. U.S. Pat. No. 3,616,231, incorporated herein by reference in its entirety. Certain uricases derived from plants or from microorganisms are more soluble in medically acceptable solvents. However, injection of the microbial enzymes quickly induces immunological responses that can lead to life-threatening allergic reactions or to inactivation and/or accelerated clearance of the uricase from the circulation. Donadio, et al., (1981); Leaustic, et al., (1983). Enzymes based on the deduced amino acid sequences of uricases from mammals, including pig and baboon, or from insects, such as, for example, Drosophila melanogaster or Drosophila pseudoobscura (Wallrath, L L, et al., (1990) Mol Cell Biol 10:5114-5127, incorporated herein by reference in its entirety), have not been suitable candidates for clinical use, due to problems of immunogenicity and insolubility at physiological pH.
Previously, investigators have used injected uricase to catalyze the conversion of uric acid to allantoin in vivo. See Pui, et al., (1997). This is the basis for the use in France and Italy of uricase from the fungus Aspergillus flavus (URICOZYME®) to prevent or temporarily correct the hyperuricemia associated with cytotoxic therapy for hematologic malignancies and to transiently reduce severe hyperuricemia in patients with gout. Potaux, L, et al., (1975) Nouv Presse Med 4:1109-1112; Legoux, R, et al., (1992) J Biol Chem 267:8565-8570; U.S. Pat. Nos. 5,382,518 and 5,541,098, each incorporated herein by reference in its entirety. Because of its short circulating lifetime, URICOZYME® requires daily injections. Furthermore, it is not well suited for long-term therapy because of its immunogenicity.
Certain uricases are useful for preparing conjugates with poly(ethylene glycol) or poly(ethylene oxide) (both referred to as PEG) to produce therapeutically efficacious forms of uricase having increased protein half-life and reduced immunogenicity. U.S. Pat. Nos. 4,179,337, 4,766,106, 4,847,325, and 6,576,235; U.S. Patent Application Publication US2003/0082786A1, each incorporated herein by reference in its entirety. Conjugates of uricase with polymers other than PEG have also been described. U.S. Pat. No. 4,460,683, incorporated herein by reference in its entirety.
In nearly all of the reported attempts to PEGylate uricase (i.e. to covalently couple PEG to uricase), the PEG is attached primarily to amino groups, including the amino-terminal residue and the available lysine residues. In the uricases commonly used, the total number of lysines in each of the four identical subunits is between 25 (Aspergillus flavus (U.S. Pat. No. 5,382,518, incorporated herein by reference in its entirety)) and 29 (pig (Wu, X, et al., (1989) Proc Natl Acad Sci USA 86:9412-9416, incorporated herein by reference in its entirety)). Some of the lysines are unavailable for PEGylation in the native conformation of the enzyme. The most common approach to reducing the inununogenicity of uricase has been to couple large numbers of strands of low molecular weight PEG. This has invariably resulted in large decreases in the enzymatic activity of the resultant conjugates.
A single intravenous injection of a preparation of Candida utilis uricase coupled to 5 kDa PEG reduced serum urate to undetectable levels in five human subjects whose average pre-injection serum urate concentration is 6.2 mg/dl, which is within the normal range. Davis, et al., (1981). The subjects were given an additional injection four weeks later, but their responses were not reported. No antibodies to uricase were detected following the second (and last) injection, using a relatively insensitive gel diffusion assay. This reference reported no results from chronic or subchronic treatments of human patients or experimental animals.
A preparation of uricase from Arthrobacter protoformiae coupled to 5 kDa PEG was used to temporarily control hyperuricemia in a single patient with lymphoma whose pre-injection serum urate concentration is 15 mg/dL. Chua, et al., (1988). Because of the critical condition of the patient and the short duration of treatment (four injections during 14 days), it is not possible to evaluate the long-term efficacy or safety of the conjugate.
Improved protection from immune recognition is enabled by modifying each uricase subunit with 2-10 strands of high molecular weight PEG (>5 kD-120 kD) Saifer, et al. (U.S. Pat. No. 6,576,235; (1994) Adv Exp Med Biol 366:377-387, each incorporated herein by reference in its entirety). This strategy enabled retention of >75% enzymatic activity of uricase from various species, following PEGylation, enhanced the circulating life of uricase, and enabled repeated injection of the enzyme without eliciting antibodies in mice and rabbits.
Hershfield and Kelly (International Patent Publication WO 00/08196; U.S. Application No. 60/095,489, incorporated herein by reference in its entirety) developed means for providing recombinant uricase proteins of mammalian species with optimal numbers of PEGylation sites. They used PCR techniques to increase the number of available lysine residues at selected points on the enzyme which is designed to enable reduced recognition by the immune system, after subsequent PEGylation, while substantially retaining the enzyme's uricolytic activity. Some of their uricase proteins are truncated at the carboxy and/or amino termini. They do not provide for directing other specific genetically-induced alterations in the protein.
In this application, the term “immunogenicity” refers to the induction of an immune response by an injected preparation of PEG-modified or unmodified uricase (the antigen), while “antigenicity” refers to the reaction of an antigen with preexisting antibodies. Collectively, antigenicity and immumunogenicity are referred to as “immunoreactivity.” In previous studies of PEG-uricase, immunoreactivity is assessed by a variety of methods, including: 1) the reaction in vitro of PEG-uricase with preformed antibodies; 2) measurements of induced antibody synthesis; and 3) accelerated clearance rates after repeated injections.
Previous attempts to eliminate the immunogenicity of uricases from several sources by coupling various numbers of strands of PEG through various linkers have met with limited success. PEG-uricases were first disclosed by F F Davis and by Y Inada and their colleagues. Davis, et al., (1978); U.S. Pat. No. 4,179,337; Nishimura, et al., (1979); Japanese Patents 55-99189 and 62-55079, each incorporated herein by reference in its entirety. The conjugate disclosed in U.S. Pat. No. 4,179,337 is synthesized by reacting uricase of unspecified origin with a 2,000-fold molar excess of 750 dalton PEG, indicating that a large number of polymer molecules is likely to have been attached to each uricase subunit. U.S. Pat. No. 4,179,337 discloses the coupling of either PEG or poly(propylene glycol) with molecular weights of 500 to 20,000 daltons, preferably about 500 to 5,000 daltons, to provide active, water-soluble, non-immunogenic conjugates of various polypeptide hormones and enzymes including oxidoreductases, of which uricase is one of three examples. In addition, U.S. Pat. No. 4,179,337 emphasizes the coupling of 10 to 100 polymer strands per molecule of enzyme, and the retention of at least 40% of enzymatic activity. No test results were reported for the extent of coupling of PEG to the available amino groups of uricase, the residual specific uricolytic activity, or the immunoreactivity of the conjugate.
In previous publications, significant decreases in uricolytic activity measured in vitro were caused by coupling various numbers of strands of PEG to uricase from Candida utilis. Coupling a large number of strands of 5 kDa PEG to porcine liver uricase gave similar results, as described in both the Chen publication and a symposium report by the same group. Chen, et al., (1981); Davis, et al., (1978).
In seven previous studies, the immunoreactivity of uricase is reported to be decreased by PEGylation and was eliminated in five other studies. In three of the latter five studies, the elimination of immunoreactivity is associated with profound decreases in uricolytic activity—to at most 15%, 28%, or 45% of the initial activity. Nishimura, et al., (1979) (15% activity); Chen, et al., (1981) (28% activity); Nishimura, et al., (1981) (45% activity). In the fourth report, PEG is reported to be coupled to 61% of the available lysine residues, but the residual specific activity is not stated. Abuchowski, et al., (1981). However, a research team that included two of the same scientists and used the same methods reported elsewhere that this extent of coupling left residual activity of only 23-28%. Chen, et al., (1981). The 1981 publications of Abuchowski et al., and Chen et al., indicate that to reduce the immunogenicity of uricase substantially, PEG must be coupled to approximately 60% of the available lysine residues. The fifth publication in which the immunoreactivity of uricase is reported to have been eliminated does not disclose the extent of PEG coupling, the residual uricolytic activity, or the nature of the PEG-protein linkage. Veronese, F M, et al., (1997) in J M Harris, et al., (Eds.), Poly(ethylene glycol) Chemistry and Biological Applications. ACS Symposium Series 680 (pp. 182-192) Washington, D.C.: American Chemical Society, incorporated herein by reference in its entirety.
Conjugation of PEG to a smaller fraction of the lysine residues in uricase reduced but did not eliminate its immunoreactivity in experimental animals. Tsuji, J, et al., (1985) Int J Immunopharmacol 7:725-730, incorporated herein by reference in its entirety (28-45% of the amino groups coupled); Yasuda, Y, et al., (1990) Chem Pharm Bull 38:2053-2056, incorporated herein by reference in its entirety (38% of the amino groups coupled). The residual uricolytic activities of the corresponding adducts ranged from <33% (Tsuji, et al.) to 60% (Yasuda, et al.) of their initial values. Tsuji, et al., synthesized PEG-uricase conjugates with 7.5 kDa and 10 kDa PEGs, in addition to 5 kDa PEG. All of the resultant conjugates are somewhat immunogenic and antigenic, while displaying markedly reduced enzymatic activities.
A PEGylated preparation of uricase from Candida utilis that is safely administered twice to each of five humans is reported to have retained only 11% of its initial activity. Davis, et al., (1981). Several years later, PEG-modified uricase from Arthrobacter protoformiae was administered four times to one patient with advanced lymphoma and severe hyperuricemia. Chua, et al., (1988). While the residual activity of that enzyme preparation was not measured, Chua, et al., demonstrated the absence of anti-uricase antibodies in the patient's serum 26 days after the first PEG-uricase injection, using an enzyme-linked immunosorbent assay (ELISA).
Previous studies of PEGylated uricase show that catalytic activity is markedly depressed by coupling a sufficient number of strands of PEG to decrease its immunoreactivity substantially. Furthermore, most previous preparations of PEG-uricase are synthesized using PEG activated with cyanuric chloride, a triazine derivative (2,4,6-trichloro-1,3,5-triazine) that has been shown to introduce new antigenic determinants and to induce the formation of antibodies in rabbits. Tsuji, et al., (1985).
Japanese Patent No. 3-148298 to A Sano, et al., incorporated herein by reference in its entirety, discloses modified proteins, including uricase, derivatized with PEG having a molecular weight of 1-12 kDa that show reduced antigenicity and “improved prolonged” action, and methods of making such derivatized peptides. However, there are no disclosures regarding strand counts, enzyme assays, biological tests or the meaning of “improved prolonged.”Japanese Patents 55-99189 and 62-55079, each incorporated herein by reference in its entirety, both to Y Inada, disclose uricase conjugates prepared with PEG-triazine or bis-PEG-triazine (denoted as PEG2), respectively. See Nishimura, et al., (1979 and 1981). In the first type of conjugate, the molecular weights of the PEGs are 2 kDa and 5 kDa, while in the second, only 5 kDa PEG is used. Nishimura, et al., (1979) reported the recovery of 15% of the uricolytic activity after modification of 43% of the available lysines with linear 5 kDa PEG, while Nishimura, et al., (1981) reported the recovery of 31% or 45% of the uricolytic activity after modification of 46% or 36% of the lysines, respectively, with PEG2.
Previously studied uricase proteins were either natural or recombinant proteins. However, studies using SDS-PAGE and/or Western techniques revealed the presence of unexpected low molecular weight peptides which appear to be degradation products and increase in frequency over time. The present invention is related to mutant recombinant uricase proteins having truncations and enhanced structural stability.